Cationic structure and charge transport in sol-gel-derived nickel-cobaltite thin films

Conductive nickel-cobaltite (NixCo3-xO4) thin films were synthesized on Al2O3 substrates by using a sol-gel process for varying Ni composition x (<= 1.0). X-ray diffraction data indicated that the polycrystalline Ni-x Co3-xO4 samples have cubic spinel (Fd (3) over barm) structure with the lattice constant increasing slightly with increasing Ni composition (by 0.5 % for x = 1.0 compared to that of Co3O4). X-ray photoelectron spectroscopy data indicated that the Ni ions in nickel-cobaltite have multi-valence (Ni2+ and Ni3+). The temperature-dependent electrical resistivity (rho) data of the nickel-cobaltite samples were measured in the 4-300 K range in comparison with Co3O4. At all temperatures (T), the resistivity of NixCo3-xO4 decreased gradually with increasing x down to rho = 1.9 x 10(-2) Omega cm for x = 1.0 at 300 K (reduction by a factor of similar to 10(-3) from that of Co3O4). The decrease in rho by the Ni doping in Co3O4 can be understood in terms of the polaronic d-electron hopping between octahedral Ni2+ and Ni3+ ions in the spinel lattice. The analysis on rho(T) for NixCo3-xO4 exhibited a linearity between ln(rho) and T-1/4 below similar to 120 K, implying a variable-range hopping. [GRAPHICS] .